The addition of caustic soda to remove metals from waste stream is known as?

The addition of caustic soda, or sodium hydroxide, to remove metals from a waste stream is best described by hydroxide precipitation. This process involves increasing the pH of the wastewater, leading to the formation of metal hydroxides, which are generally insoluble. When the pH is raised, metals such as copper or lead react with hydroxide ions to form a precipitate (metal hydroxide), which can then be removed from the liquid phase through sedimentation or filtration.

This method is particularly effective for many toxic heavy metals commonly found in industrial effluents. By allowing the metal ions to precipitate out of solution, the amount of harmful substances in the waste stream is substantially reduced, contributing to cleaner discharge and achieving compliance with environmental regulations.

In contrast, the other options involve different chemical mechanisms. Oxidation-reduction reactions pertain to electron transfer processes and are not specifically aimed at precipitating metals. Coagulation involves the aggregation of particles in water treatment processes but does not directly relate to the specific removal of dissolved metal ions through precipitation. Chelation refers to the formation of complex ions with metal ions in solution, which usually stabilizes them and prevents precipitation, rather than facilitating their removal.